public int compare(Person firstPerson, Person secondPerson) {
   String personName1 = firstPerson.getFirstName().toUpperCase();
   String personName2 = secondPerson.getFirstName().toUpperCase();
   // ascending order
   return personName1.compareTo(personName2);
   // descending order
   // return fruitName2.compareTo(fruitName1);
 }
 public static void displayList(Person[] persons) {
   for (int i = 0; i < 4; i++) {
     Person person = persons[i];
     String lastName = person.getLastName();
     String firstName = person.getFirstName();
     int age = person.getAge();
     System.out.println(lastName + ", " + firstName + ". Age:" + age);
   }
   System.out.println();
 }